3Mg/Mg2Sn anodes with unprecedented electrochemical performance towards viable magnesium-ion batteries

The development of high-performance Mg-alloying anodes operable in conventional electrolytes could be a critical breakthrough for the viability of magnesium-ion batteries (MIBs). Herein, we show that 3Mg/Mg2Sn can be used as an anode with unprecedented high performance in conventional electrolytes. The as-made 3Mg/Mg2Sn consists of crystalline Mg-rich (c-Mg), amorphous Mg-rich (a-Mg), and intermetallic Mg2Sn phases. During the 1st de-magnesiation of 3Mg/Mg2Sn in Mg(HMDS)2:MgCl2 (HMDS = hexamethyldisilazide), c-Mg is first dealloyed. After the complete dissolution of c-Mg, a-Mg and intermetallic Mg2Sn are then de-magnesiated at slightly higher voltages, resulting in a capacity of 1243 mA h g-1 (3.9 Mg2+) at 100 mA g-1. Subsequent charge/discharge reversibly delivers ca. 805 mA h g-1 (2.5 Mg2+) with no re-formation of c-Mg, which indicates that the reversibility comes mostly from Mg2Sn (Mg2Sn ⇆ Sn) with an additional contribution from a-Mg (0.5 Mg2+). Though not involved in the reversible process, the dissolution of c-Mg generates the micro-to-macropores, which contribute to the excellent rate-capability and cyclability of 3Mg/Mg2Sn. The Mg2+ ions, irreversibly released from 3Mg/Mg2Sn, also compensate for a decrease in the Mg2+ concentration, which can occur when a full-cell is constructed with an Mg2+-trapping Mo6S8 cathode. Furthermore, once dealloyed in Mg(HMDS)2:MgCl2, the 3Mg/Mg2Sn becomes compatible with various conventional electrolytes.